Due to the fact that most deep-sea benthic species are deposit feeders (Sanders and Hessler, 1969), the locally qualitatively and quantitatively, variable import of POM from the ocean’s surface waters plays a crucial role for macro-, meio-, and microorganisms living in deep surface sediments (Gooday and Turley, 1990). Mainly consisting of phytoplankton, marine snow, fecal pellets, (dead) zooplankton and molts, this material undergoes different steps of degradation during its passage from the photic, epipelagic (0–200 mbsl), through the mesopelagic (200–1,000 mbsl) to the actual deep-sea zones, in particular the bathypelagic (1,000–4,000 mbsl), the abyssal (4,000–6,000 mbsl), and the hadal zone (6,000–11,000 mbsl).
Depending on the residence time in the water column, the bioavailable part of POM finally reaching the deep-seafloor may be small (De La Rocha and Passow, 2007 and references therein). The refractory remainders such as animal skeletons are continuously accumulating at the seafloor and turn into deeply buried sediment over time, thereby representing the largest global reservoir organic carbon (Parkes et al., 2000 and references therein).
2.4DEEP-SEA SEDIMENT TYPES
Grain size (Gray, 1974) and sediment heterogeneity (Etter and Grassle, 1992) may additionally govern community composition and distribution of macro-, meio-, and microorganisms in deep-sea benthic environments. In relation to their basic sources, deep-sea sediments may be biogenic (POM from pelagic primary production, benthic in-situ production), lithogenous (terrestrial weathering and transport by wind and rivers), hydrogenous (precipitation from seawater or pore water), volcanic, or cosmic (Seibold and Berger, 1996). According to grain size and settling velocity, lithogenous gravel and sandy fractions usually are deposited along the coast, while silt and clay are transported farther offshore through waves and currents, hence dominating the basically biogenous deep-sea
sediments. Regional deviations may be linked to currents, downslope slides, submarine canyon dynamics, or to a release of ice-trapped rock material in polar waters (e.g., Ramseier et al., 2001). Covering almost one-half of the shelves and more than half of the deep ocean bottom, biogenous sediments mainly consist of calcite, aragonite, opal, and calcium phosphate, originating from foraminifera, diatoms, and radiolarians (Hay et al., 1988).
2.5DEEP BIOSPHERE OF DEEP-SEA SEDIMENTS
Microbiological studies on sediment cores collected during several cruises of the Deep Sea Drilling Project (DSDP), the Ocean Drilling Program (ODP), and the Integrated Ocean Drilling Program (IODP) gave evidence for the presence of complex microbial communities in deeply buried marine sediments down to several hundred meters below seafloor (e.g., Whelan et al., 1986; Parkes et al., 1994; Roussel et al., 2008). Most striking, new insights into subsurface microbiology were gained during the ODP cruise Leg 201 to the equatorial Pacific Ocean and the continental margin of Peru, including sites recognized as most typical for oceanic subsurface environments (D’Hondt et al., 2002). A large fraction of the sub-seafloor bacteria has been proven to be alive and culturable, displaying turnover rates (based on sulfate reduction as dominating mineral process at these sites) comparable to surface sediment communities (D’Hondt et al., 2004; Schippers et al., 2005).
After a logarithmic decline within the uppermost 6 meters below seafloor (mbsf) (Parkes et al., 1994) to about 40 mbsf (Schippers et al., 2005), bacterial cells have proven to be more or less evenly distributed down to several hundred mbsf. Local peaks within these deeply buried sediments seem to mirror sulfate (diffusing from crustal fluids) and methane (from in-situ production) concentration shifts (Engelen et al., 2008). However, published variations of absolute cell numbers (by a factor of up to 3) have to be treated with caution: varying estimations not only depend on the geochemical conditions at the respective sampling sites,
but also on the enumeration techniques applied. Calculations based on early results revealed that sub-seafloor sediments comprise –at least – half of all prokaryotic cells and up to one-third of the living biomass on Earth pointing to a slow-growing strategy of high biomass in areas of low-energy flux (Whitman et al., 1998).
The prokaryotic community in deeply buried sediments can not exclusively be traced back to contaminations from biologically active surface layers or reactivation of spores and dormant cells (Parkes et al., 2000 and references therein).
Porewater chemistry data obtained from sites throughout the world’s oceans (ODP, DSDP) showed that sulfate reduction, methanogenesis, and fermentation are the principal degradative metabolic processes in subsurface sediments. These results give evidence for significant lower metabolic rates for the subsurface compared to the surface biosphere and for methanogenesis becoming more important the more sulfate gets depleted with increasing sediment depth (D’Hondt et al., 2002 and references therein).
2.6WINDOWS TO THE SUBSURFACE BIOSPHERE
2.6.1 Hydrothermal vents
The discovery of the “ocean vents” near Galapagos Island (Corliss et al., 1979) was the first proof for the active movement of the gigantic oceanic plates of the Earth’s crust creating series of cracks in the ocean floor, teeming with life. At these discharge areas, hydrothermal fluids with temperatures of more than 400°C (Haase et al., 2007) mix up with the cold ocean seawater, resulting in a precipitation of dissolved metals and in the formation of characteristic chimneys over time. Iron and sulfide precipitates turn the smokers black (“black smokers,”
Figure 1), while barium, calcium, and silicon minerals result in “white smokers.” Thermal precipitation and/or direct magma degassing of H2, H2S, CH4, CO, and CO2 in combination with oxygen as electron acceptor provide enough energy to support a highly productive and
physiologically diverse chemoautotrophic microbial community (Reysenbach and Shock, 2002).
As the highly diverse and dense hot vent macrofauna (e.g., vestimentiferan tubeworms, bivalve mollusks, provannid gastropods, alvinellid polychaete, and bresiliid shrimps) cannot feed on the released chemicals themselves, they either feed on chemoautotrophic microbes or host them as symbionts. The predominant endosymbionts are mesophilic to moderately thermophilic chemoautotrophs (mostly Gammaproteobacteria), whereas most episymbionts belong to the Epsilonproteobacteria, which can oxidize H2 and sulfur compounds while reducing oxygen, nitrate, and sulfur compounds (for review, see Nakagawa and Takai, 2008).
The vent habitat proved to harbor methanogens (Methanococcus), sulfate-reducers (Archaeoglobus), and facultative autotrophs and heterotrophs such as the thermophilic aerobic Thermus and Bacillus (Harmsen et al., 1997) or, for example, Thermococcus, Phyrococcus, Desulfurococcus (Prieur et al., 1995; Teske et al., 2000; Nercessian et al., 2003; Schrenk et al., 2003). Generally detected archaeal phylotypes were affiliated with hyperthermophilicCrenarchaeota, Euryarchaeota Group I, II, III (Takai and Horikoshi, 1999) and the “Deep-sea Hydrothermal Vent Euryarchaeotal Group” (Hoek et al., 2003).
2.6.2 Cold seeps and mud volcanoes
Just a few years after the discovery of the hydrothermal vent systems, cold seep ecosystems were reported from active and passive continental margins and subduction zones all over the world (Aharon, 1994 and references therein).
High-pressure, low oxygen and low-temperature conditions favour the formation of marine gas hydrates. In the subsurface realm, such gas reservoirs are stored in a crystalline form, whereas they get, dissolved in pore waters and finally leave the sediment surface in gaseous form. High fluxes of methane, sulfide, and other reduced elements characterize these
ecosystems such as cold seeps, hydrocarbon vents and mud volcanoes, often leaving mineral precipitation in their immediate surroundings. Coupled to sulfate reduction, rich bacterial and archaeal communities perform anaerobic oxidation of hydrocarbons, but predominately of methane (Boetius et al., 2000; Borowski et al., 2000; Treude et al., 2005). Conversion of methane is mainly mediated by two different groups of anaerobic methanotrophic archaea (ANME-I and ANME-II) (Nauhaus et al., 2005), forming syntrophic consortia with the sulfate-reducing bacteria (SRB) Desulfosarcina and Desulfococcus (Hinrichs et al., 1999;
Boetius et al., 2000; Michaelis et al., 2002; Knittel et al., 2003).
The methane-emitting Haakon Mosby Mud Volcano (HMMV, Barents Sea) has shown to harbor three key communities in methane conversion such as aerobic, methanotrophic bacteria (Methylococcales), anaerobic methanotrophic archaea (ANME-2) thriving below siboglinid tubeworms, and a previously undescribed clade of archaea (ANME-3) associated with bacterial mats (Niemann et al., 2006). Similarly, some cold seeps on the deeper Black Sea shelf are characterized by intense methane bubble discharge, mainly related to microbial methanogenesis (Pape et al., 2008 and references therein). Diffuse gas seeps in more shallow, oxic Black Sea waters often exhibit a netlike coverage of microbial mats similar to Beggiatoa-mats observed at HMMV (Figure 2). Beggiatoa spp. are discussed as keystone members of seep communities owing to their ability to (directly and indirectly) influence the metabolic activity of d-Proteobacteria, Planctomycetales, and ANME archaea by providing sulfate and ammonia as reactants (Mills et al., 2004).
The question remains, to which extent such seep systems influence the global methane cycle, as the quantification of bubble dissolution and/or the release of methane- rich pore fluids from the sediment into the hydrosphere is difficult to achieve (Vogt et al., 1999; Reeburgh, 2007).
Niemann et al. (2006) estimated that methanotrophy at active marine mud volcanoes
consumes less than 40% of the total methane flux, due to limitations of the relevant electron acceptors in the upward flowing, sulphate- and oxygen-free fluids.
2.7DEEP BIOSPHERE OF THE OCEANIC CRUST
The fact that microorganisms are present in the subsurface realm had been reported decades ago in terrestrial subsurface environments (Farrell and Turner, 1931; Lipman, 1931). Early drilling operations performed for commercial purposes such as mining, oil and hot water recovery, and the search for underground waste repositories reported on the existence of a large community of microorganisms obviously involved in geochemical processes in the deep biosphere (Gold, 1992; Pedersen, 1993 and references therein). Hence, it was only in the early 1990s that scientists started to focus on the investigation of prospering life beneath the Earth’s crust, thanks to a chance encounter of a deep oceanic, volcanic eruption during a dive onboard the submersible Alvin, releasing white microbial bulk mats (Haymon et al., 1993). The upper layers of the oceanic crust are characterized by high basaltic porosity, hosting a vast hydrothermal reservoir (Johnson and Pruis, 2003) inhabited by a microbial community composed of species that are also found in deep-sea waters, sediments, and the deep oceanic crust (Thorseth et al., 2001; Huber et al., 2006). Among the most prominent anaerobic thermophiles indigenous for the oceanic crust, the Ammonifex group of bacteria (Nakagawa et al., 2006) or groups within Crenarchaeota, Euryarchaeota, and Korarchaeota (Ehrhardt et al., 2007). Since 3.5 billion years, basaltassociated glass textures and vesicular cavities within the basaltic matrix provide niches for microbial colonization (Furnes et al., 2004; Peckmann et al., 2008). For instance, the fossil record of the oceanic crust even gives evidence for a previous fungal life in deep ocean basaltic rocks (Schumann et al., 2004).
Much effort has been put into the investigation of the deep biosphere of the deep sea during the past 20 years. However, we still are neither aware of the final composition of the living
subsurface community, nor of its interrelationship to, for example, crustal fluid-derived compounds, nor of its global impact.
2.8FIGURES
Figure 1
Black smoker “Candelabra,” Logachev hydrothermal field, Mid-Atlantic-Ridge (3,000 m water depth). (By courtesy of MARUM, Center for Marine Environmental Sciences, Bremen, Germany.)
Figure 2
Shallowwater seep area “GHOSTDABS-field,” Ukrainian shelf, Black Sea. White nets are constructed of sulfide oxidizing bacteria (“Beggiatoa”). (By courtesy of Karin Hissmann and Jürgen Schauer, Jago Team, Leibniz-Institut für Meereswissenschaften (IFM-GEOMAR) Kiel, Germany.)
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